Turbulence statistics downstream of a vorticity generator at low Reynolds numbers

Abstract : Vortex generators (VGs) are inserted in turbulent pipe flows in order to improve mixing and heat and mass transfer while a moderate pressure drop is maintained. The purpose of the present study is to contribute to the elaboration of scaling laws for the turbulence decay downstream a row of VGs. This knowledge will help in the design of such systems, especially for optimal geometry and spacing of the VG. The experimental study is carried out using laser Doppler anemometry at different locations downstream of the row of VGs so as to probe the streamwise velocity field. The Taylor microscale Reynolds number Reλ ranges between 15 and 80 so that, for the lowest flow rates, fully developed turbulence conditions are not fulfilled. Comparison of the integral length scale to data in the open literature shows that the conventional scaling laws at the dissipative scale are fairly assessed. It is shown that the turbulence macroscale increases in the streamwise direction and is scaled by the VG dimensions. The normalized turbulent energy dissipation rate has values between 0.5 and 2.8, with −1 power-law decay as a function of the Taylor microscale Reynolds number. This observation is consistent with previous findings using direct numerical simulations (DNS). The streamwise variation of the turbulence energy dissipation rate shows an exponential decay; it reaches an asymptotic value after a distance of about 6 times the VG height.